BUG/MEDIUM: checks: fix slow start regression after fix attempt

Commit 2e99390 (BUG/MEDIUM: checks: fix slowstart behaviour when server
tracking is in use) moved the slowstart task initialization within the
health check code and leaves it unset when checks are disabled. The
problem is that it's possible to trigger slowstart from the CLI by
issuing "disable server XXX / enable server XXX" even when checks are
disabled. The result is a crash when trying to wake up the slowstart
task of that server.

Move the task initialization earlier so that it is done even if the
checks are disabled.

This patch should be backported to 1.4 since the commit above was
backported there.
BUG/MAJOR: server: weight calculation fails for map-based algorithms

A crash was reported by Igor at owind when changing a server's weight
on the CLI. Lukas Tribus could reproduce a related bug where setting
a server's weight would result in the new weight being multiplied by
the initial one. The two bugs are the same.

The incorrect weight calculation results in the total farm weight being
larger than what was initially allocated, causing the map index to be out
of bounds on some hashes. It's easy to reproduce using "balance url_param"
with a variable param, or with "balance static-rr".

It appears that the calculation is made at many places and is not always
right and not always wrong the same way. Thus, this patch introduces a
new function "server_recalc_eweight()" which is dedicated to this task
of computing ->eweight from many other elements including uweight and
current time (for slowstart), and all users now switch to use this
function.

The patch is a bit large but the code was not trivially fixable in a way
that could guarantee this situation would not occur anymore. The fix is
much more readable and has been verified to work with all algorithms,
with both consistent and map-based hashes, and even with static-rr.

Slowstart was tested as well, just like enable/disable server.

The same bug is very likely present in 1.4 as well, so the patch will
probably need to be backported eventhough it will not apply as-is.

Thanks to Lukas and Igor for the information they provided to reproduce it.
MINOR: stats: report correct throttling percentage for servers in slowstart

The column used to report the throttle percentage when a server is in
slowstart is based on the time only. This is wrong, because server weights
in slowstart are updated at most once a second, so the reported value is
wrong at least fo rone second during each step, which means all the time
when using short delays (< 20s).

The second point is that it's disturbing to see a weight < 100% without
any throttle at the end of the period (during the last second), because
the effective weight has not yet been updated.

Instead, we now compute the exact ratio between eweight and uweight and
report it. It's always accurate and describes the value being used instead
of using only the date.

It can be backported to 1.4 though it's not particularly important.